Open path laser/optical communication systems and methods utilizing wavelengths between atmospheric and gaseous absorption lines

ABSTRACT

An open-path optical communication system has either optical or laser sources and communicates between the source and a detector. In a first embodiment, the laser source includes a gas cell in the laser cavity to regulate laser wavelengths based on the minimum absorption between spectral lines of the gas in the cell. The laser is tuned close to a minimum absorption wavelength and the minimum absorption line locks the laser wavelength to the minimum position. In a second embodiment, the strong absorption lines of a gas in a gas cell positioned at a receiver site are used to provide channel isolation of the receiver. In a third embodiment, an atmospheric gas provides the channel isolation. In the fourth embodiment, individual wavelength channels are positioned between the absorption lines of atmospheric or non-atmospheric gases to prevent cross-talk between adjacent channels.

RELATED APPLICATIONS

This present disclosure is a division of a co-pending disclosure of thesame title by the same inventors, filed Jun. 30, 2006, bearing Ser. No.10/604,193, which is a continuation of International ApplicationPCT/US02/02865 filed Jan. 30, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for relaying information between twoor more points using laser or optically generated conductive transportmeans through the atmosphere to convey the information.

2. Description of the Prior Art

Radio waves or other optical transport waves and less sophisticatedsystems using wires, fiber-optic cables, and other physical transportmeans have been employed to relay information between two or morepoints. Radio transmissions are heavily dependent upon atmosphericconditions, and fiber-optic cables or other physical transport means areexpensive.

Communication systems are therefore needed that do not rely upon radiotransmissions, fiber-optic cables, or other physical transport means.

An open path laser beam communication system overcomes many of theproblems associated with prior art communication systems, but has itsown limitations. Specifically, meaningful multi-channel WDM (WavelengthDivision Multiplexed), open-path laser communication requires that eachlaser be at a different wavelength, and possibly tunable. Tunable lasersrequire tuning elements such as gratings, etalons, and the like. Thesetuning elements can tune wavelengths to a precision of 1 nm (1 cm⁻¹).Finer wavelength control is difficult and expensive to maintain usingconventional techniques. Typically, a laser wavelength is locked to apeak maximum absorption of a gas that is internal or external to thelaser cavity, or to an external wavelength spectrometer instrument.

Wavelength-controlled laser/optical open-path communication systemsoften have difficulty in controlling the laser wavelength to that of thereceiver optical bandwidth. Moreover, multi-wavelength channel crosstalk is often a problem because it is difficult to produce very narrow(<10 nm) optical filters.

However, in view of the prior art considered as a whole at the time thepresent invention was made, it was not obvious to those of ordinaryskill in the art how the limitations of the known systems and methodscould be overcome.

SUMMARY OF THE INVENTION

The long-standing but heretofore unfulfilled need for a laser that doesnot require fine wavelength control is now provided by a system thatincorporates, in a first embodiment, a laser having a non-filterbandwidth-defining structure preferably in the form of a gas cellpositioned in the laser cavity.

The gas cell positioned inside the laser cavity enables the laser tooperate at wavelengths between the maximum absorption line wavelengthsof the gas in the cell. This is the spectral position or wavelength thathas higher gain, i.e., the lowest absorption loss. Accordingly, the onlytuning required is a coarse wavelength control such as a grating prism.The need for additional equipment providing fine wavelength control istherefore obviated.

More particularly, a gas cell containing gas with individualvibrational-rotation line spectra is positioned inside a tunable lasercavity having a resonance wavelength and the cavity resonance wavelengthis positioned between adjacent absorption lines of the gas. The lasertherefore operates at an absorption minimum that occurs between theabsorption lines and the laser wavelength is locked to an absolutewavelength defined by the gas. Advantageously, the maximum absorptionbands act as filters for the laser wavelength output.

In a second embodiment, the need to control laser wavelength to that ofa receiver optical bandwidth is fulfilled by harnessing strong opticalabsorption lines in a preselected gas in a gas cell positioned at areceiver site upstream of the receiver. This provides an absolutewavelength reference or control for laser wavelength andreceiver/detector optical bandwidth in an open path laser/opticalcommunication system.

In a third embodiment, the need to control laser wavelength to that ofthe receiver optical bandwidth is fulfilled by harnessing the strongoptical absorption lines in the atmosphere that are due to atmosphericoxygen.

A fourth embodiment discloses a novel method for controlling awavelength-controlled laser to the optical bandwidth of a receiver meansin an open-path communication system. The laser is tuned so that itlases at minimum absorption wavelengths positioned between strongrotational-vibrational spectral absorption lines in atmospheric gases.The strong absorption lines provide optical guard channels that preventcross-talk between adjacent wavelength channels. An absorption lineminimum locks the laser to the minimum absorption position and relianceupon optical bandwidth filters in a receiver channel is reduced. Anexternal tuning means is employed to tune the laser to within a fewnanometers of the minimum absorption wavelength so that it lases at theminimum spectral absorption lines where the laser cavity has maximumgain. Positioning an absorbing gas cell in the laser cavity of the laserforces the laser output to operate at wavelengths at the minimum of thespectral absorption lines.

An important object of this invention is to provide a tunable laserhaving a gas cell positioned within the laser cavity.

A more specific object is to provide a laser communication system lasersource that operates at known absolute wavelengths defined by theminimum of absorption between gaseous absorption lines.

Another object is to provide a laser system that operates withinexpensive coarse tuning equipment and does not require expensive finetuning equipment.

Still another object is to optimize the wavelengths used in a lasercommunication system to select wavelengths that use the absorptioncharacteristics of the atmosphere or external gas cell to enhance theperformance of a laser communication system detection device.

Another important object is to provide a method for providing opticalguard channels that prevent cross-talk between adjacent wavelengthchannels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a laser beam source where a beam is lockedon a wavelength determined by the absorption characteristics of a gascell;

FIG. 2 is a graph depicting a portion of the oxygen absorption spectrum;

FIG. 3A is a schematic view of a laser system having an atmospheric gascell for filtering wavelengths to the absorption characteristics of aselected gas; and

FIG. 3B is a schematic view of a variation of the system depicted inFIG. 3A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, it will there be seen that a laser sourcehaving a gas cell as an integral part of the laser mechanism, i.e.,within the laser cavity is denoted as a whole by the reference numeral10. A beam from continuous-wave laser 12 sequentially passes through gascell 14 and laser cavity output mirror 16. Because of the gas cell inthe laser cavity, the resultant beam is locked to the actual minimumabsorption bands of the gas selected in the cell.

Laser 12 is preferably provided in the form of a continuous waveexternal cavity GaAs diode laser having a wavelength of 0.76 μm. Thelaser assembly further includes back laser cavity mirror 11 and a coarsetuning current control means 13. Lasers of types other than continuouswave can also have wavelengths selected in this manner.

Gas cell 14 is preferably ten meters in optical path length and containsoxygen at a pressure of one atmosphere.

Gas cell 14 contains gas with individual vibrational-rotation linespectra. Significantly, it is used within a tunable laser cavity asdepicted in FIG. 1. Coarse wavelength tuning is conducted usinggratings, prisms, or etalons to position the cavity resonant wavelengthbetween adjacent absorption lines of the gas. The laser operates at theabsorption minimum occurring between the gas absorption lines, thuslocking the laser wavelengths to the absolutes wavelength defined by thegas. This locked wavelength is at the minimum absorption of the gas asopposed to the maximum absorption of the gas as in the prior art methodsusing a gas cell positioned internally or externally to the lasercavity.

FIG. 2 depicts a spectrum of oxygen gas with absorption bands 20 andminimum absorption bands 22. In practice, the laser output in thissystem is now locked onto minimum absorption spectral region 22, andmaximum absorption lines 20 act as a filter for the laser wavelengthoutput. The novel system thus enables use of a less sophisticated, andless costly, laser and wavelength monitors relative to prior artreflection grating systems or an active cavity length turning device.

The novel system is also useful for wavelength control of fiber opticlaser communication systems, point-to-point open-path lasercommunication systems and in open-path laser communication systems wherebarriers intervene between the transmitting and receiving communicationdevices such as those described in co-pending patent application filedJan. 30, 2002, entitled Open-Path/Free-Space Optical CommunicationSystem And Method Using Reflected or Backscattered Light, by the sameinventor, which disclosure is hereby incorporated by reference into thisdisclosure.

Two embodiments of the invention are depicted in FIG. 3A, saidembodiments being the second and third embodiments of the invention.

In the second embodiment, laser system 30 includes a gas cell 34positioned upstream of a detector means to provide channel isolation atthe receiver. The gas cell bandwidth system pre-filters the light at thedetector means. Laser source 32, which can be a sole or multiple lasersource, emits a beam of light 33 that is passed through gas cell 34prior to passing through a moderate-bandwidth optical filter 36 toimpinge upon a detector means such as detectors 38 and 39. Thiseliminates the use of highly expensive narrow-bandwidth optical filtersbecause the gas in cell 34 acts in substantially the same way as the gasin the laser source of the first-described embodiment to filter thebandwidths to narrow bands defined by the absorption characteristics ofthe gas in said cell 34. The gas may be any atmospheric gas such asoxygen, carbon dioxide, nitrogen, etc., or it may be any non-atmosphericgas such as HI, HF, or even benzene. The use of any specific gas is thechoice of one of ordinary skill in the art, as the selection is based onthe specific absorption lines of that gas and specific applicationrequirements.

This novel use of gas cell 34 is further advantageous because individualwavelength channels are formed between the absorption lines of the gasin the gas cell so that said absorption lines block each channel fromits adjacent channel.

Beam splitter 40 enables a user to set the detection wavelengths in aplurality of detectors to various discrete wavelengths as defined by thegas in the intervening cell. Again, less costly filters such asmoderate-bandwidth optical filters are used at the detector site becauseunwanted nearby wavelengths are filtered out by the absorptioncharacteristics of the gas by virtue of channel isolation.

More particularly, tunable laser source 32 is preferably a GaAs laserhaving λ₁=0.7665 μm and λ₂=0.7660 μm. The effective optical bandwidth ofgas cell 34 is 5 cm⁻¹.

In the third embodiment, also depicted in FIG. 3A, light beam 33 followsan open path as in the incorporated disclosure and does not pass throughgas cell 34. Light beam 33 does not pass through gas cell 34 but insteadundergoes atmospheric absorption with oxygen. The channel isolation ofthe receiver thereby provided is similar to that of the secondembodiment. This use of the atmosphere causes the formation ofindividual wavelength channels between the absorption lines of theoxygen so that said absorption lines block each channel from itsadjacent channel.

In both the second and third embodiments, as illustrated in FIG. 3A,beam splitter 40 divides the beam so that part of it passes throughoptical filter 36 and another part of it passes through optical filter37. The part of beam 33 that passes through optical filter 37 impingesupon optical detector 38 and the part thereof that passes throughoptical filter 36 impinges upon optical detector 39. Each optical filteris of the thin-film type and has a bandwidth of 15 cm⁻¹.

FIG. 3B depicts a variation of the second embodiment. Laser source 30 ais a GaAIAs diode laser where λ₁=1.567 μm (6381 cm⁻¹) and λ₂=1.566 μm(6384 cm⁻¹). Gas cell 34 a is ten meters in optical path length andcontains carbon monoxide at one atmosphere of pressure. Beam splitter 40a divides the beam so that it passes through optical filters 36 a and 37a, each of which has a 6 cm⁻¹ bandwidth. As in the second and thirdembodiments, the arrangement of FIG. 3B provides channel isolation atthe receiver site of an open path laser system, and individualwavelength channels are blocked from their adjacent wavelength channelsby the absorption lines of the gas in gas cell 34 a.

The fourth embodiment of this invention harnesses the observation thatmany atmospheric and non-atmospheric gases have strong, individual anddistinct rotational-vibrational spectral absorption lines. Theseabsorption lines are used advantageously within a tunable laser cavityto force or control the laser to operate at wavelengths between theabsorption line centers, i.e., where the transmission is highest. For amoderately tunable diode or other type of laser, an external tuningmeans such as grating, diode current, or temperature may be used to tunethe laser close (within a few nanometers) to minimum absorptionwavelength. The absorption line minimum locks the laser wavelength tothe minimum position, i.e., the laser will lase at the minimum of thespectral absorption, where the laser cavity has the highest gain.

Multiple wavelength optical/laser open path communication systemsoperating through the atmosphere can operate at many simultaneouswavelength channels. If the individual wavelength channels occur betweenthe absorption lines of non-atmospheric or atmospheric gases, then eachchannel is blocked from drifting (in wavelength) into the adjacentchannel by the adjacent strong absorption line. As such, the strongabsorption lines act like adjacent WDM optical bandwidth filters orFabry-Perot transmission modes. The resultant optical blocking filterswhich still need to be used are wider in wavelength bandwidths and thusless expensive. For example, the approximately thirty (30) CO₂ gasabsorption lines near 1.575 μm are separated by 2 to 3 cm⁻¹, i.e., about2 to 3 Å or 0.2 to 0.3 nm. The narrow optical bandwidth filters wouldhave a passband of 2 to 3 Å instead of a narrower bandwidth.

The fourth embodiment of this invention is therefore understood to be amethod for preventing cross-talk between adjacent wavelength channels.The novel method includes the step of controlling awavelength-controlled laser to the optical bandwidth of a receiver meansin an open-path communication system by tuning the laser so that itlases at minimum absorption wavelengths positioned between strongrotational-vibrational spectral absorption lines in atmospheric gases.Strong absorption lines therefore provide optical guard channels thatprevent the cross-talk. An absorption line minimum locks the laser tothe minimum absorption position and reliance upon optical bandwidthfilters in a receiver channel is reduced. This enables the use of lessexpensive optical bandwidth filters in the receiver/detector channel.

An external tuning means is used to tune the laser to within a fewnanometers of the minimum absorption wavelength so that it lases at theminimum spectral absorption lines where the laser cavity has maximumgain. An absorbing gas cell positioned in the laser cavity of the laserforces the laser output to operate at wavelengths at the minimum of thespectral absorption lines.

The systems that use the gas cell absorption are appropriate for theopen path systems of the incorporated disclosure, but they also may beused in direct point-to-point laser/optical systems and fiber opticlaser/optical communication systems.

It may be appreciated by one skilled in the art that additionalembodiments may be contemplated, including alternate embodiments of thelaser or optical sources and the detectors.

In the foregoing description, certain terms have been used for brevity,clarity and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwords are used for description purposes herein and are intended to bebroadly construed. Moreover, the embodiments of the apparatusillustrated and described herein are by way of example, and the scope ofthe invention is not limited to the exact details of construction.

Having now described the invention, the construction, the operation anduse of preferred embodiments thereof, and the advantageous new anduseful results obtained thereby, the new and useful constructions, andreasonable mechanical equivalents thereof obvious to those skilled inthe art, are set forth in the appended claims.

1. A method for providing channel isolation at a receiver means in anopen-path communication system, comprising the steps of: providing a gascell having gases therein selected from the group of gases includingatmospheric gases and non-atmospheric gases, said gases havingabsorption lines; providing a detector means in said receiver means;positioning said gas cell in said receiver means upstream of saiddetector; whereby unwanted light is pre-filtered; whereby absorptionlines in said gas provide wavelength control for laser wavelength andfor the optical bandwidth of the receiver; whereby channel isolation isprovided at the receiver; and whereby individual wavelength channels areformed between the absorption lines of said gases so that each channelis blocked by said absorption lines from its adjacent channel.
 2. Themethod of claim 1, further comprising the steps of: providing saiddetector means in the form of a first and a second detector; positioninga beam splitter between said gas cell and said first detector;positioning said beam splitter between said gas cell and said seconddetector; setting detector wavelengths for said first and seconddetectors to discrete wavelengths as defined by the gas in said gascell; whereby moderate bandwidth optical filters are used at a detectorsite because unwanted wavelengths are filtered out by the absorptioncharacteristics of the gas in the gas cell.
 3. The method of claim 2,further comprising the steps of: positioning a first moderate bandwidthoptical filter between said beam splitter and said first detector andpositioning a second moderate bandwidth optical filter between said beamsplitter and said second detector.
 4. A method for providing channelisolation at a receiver means in an open-path communication system,comprising the steps of: positioning a laser cavity resonance wavelengthsubstantially mid-way between adjacent absorption lines of a preselectedatmospheric gas; providing a detector means in said receiver; providingsaid detector means in the form of a first and second detector;positioning a beam splitter between a gas cell and said first detector;positioning said beam splitter between said gas cell and said seconddetector; setting detector wavelengths for said first and seconddetectors to discrete wavelengths as defined by the preselectedatmospheric gas; positioning a first moderate bandwidth optical filterbetween said beam splitter and said first detector and positioning asecond moderate bandwidth optical filter between said beam splitter andsaid second detector; and tuning a laser source to within a fewnanometers of the minimum absorption wavelength so that it lases at theminimum spectral absorption lines where said laser cavity has maximumgain; whereby absorption lines in said preselected atmospheric gasprovide wavelength control for laser wavelength and for the opticalbandwidth of the receiver; whereby moderate bandwidth optical filtersare used at a detector site because unwanted wavelengths are filteredout by the absorption characteristics of the preselected atmosphericgas; and whereby channel isolation is provided at the receiver.
 5. Themethod of claim 4, further comprising the steps of: positioning a firstmoderate bandwidth optical filter between said beam splitter and saidfirst detector and positioning a second moderate bandwidth opticalfilter between said beam splitter and said second detector; and tuning alaser source to within a few nanometers of the minimum absorptionwavelength so that it lases at the minimum spectral absorption lineswhere said laser cavity has maximum gain.
 6. A method for preventingcross-talk between adjacent wavelength channels, comprising the stepsof: controlling a wavelength-controlled laser to the optical bandwidthof a receiver means in an open-path communication system by tuning thelaser so that it lases at minimum absorption wavelengths positionedbetween strong rotational-vibrational spectral absorption lines inatmospheric gases; said strong absorption lines providing optical guardchannels that prevent the cross-talk; whereby an absorption line minimumlocks the laser to the minimum absorption position and reliance uponoptical bandwidth filters in a receiver channel is reduced.
 7. A methodfor preventing cross-talk between adjacent wavelength channels,comprising the steps of: providing a gas cell having atmospheric gaseswith strong absorption lines therein; positioning said gas cell in areceiver upstream of a detector; said strong absorption lines providingoptical guard channels that prevent the cross-talk; whereby anabsorption line minimum locks the laser to the minimum absorptionposition; and whereby reliance upon optical bandwidth filters in areceiver channel is reduced.